Major depressive disorder is a serious neuropsychiatric disease that leads to significant impairment in social functioning and increased morbidity and mortality. Low vitamin D (25-OH D) levels have been hypothesized to contribute to the pathophysiology of MDD. To investigate the therapeutic role of vitamin D in MDD, we recruited 62 male and female patients diagnosed with MDD and randomized them into two groups: the first group (49 patients) received vitamin D supplementation as cholecalciferol vitamin D3 (50,000 I.U.) for 3months, in addition to standard of care (SOC) which included pharmacological treatment and psychological support, and the second group (13 patients) received only SOC without vitamin D supplementation for 3months. The Beck depression inventory (BDI) scale was used to assess the severity of MDD symptoms. Immunoassays were utilized to determine levels of serum vitamin D3 and serotonin in all patients. The results showed significant gender differences; female patients showed the most improvement in their depressive symptoms after 3-month vitamin D supplementation. Females with moderate, severe, and extreme depression had significantly lower BDI scores after vitamin D treatment (p < 0.05). Among males, only those diagnosed with severe depression showed significant improvement in their BDI scores (p < 0.05). Serum serotonin levels were significantly increased after vitamin D supplementation compared to baseline in both male and female patients. No significant changes in other biochemical parameters were detected between the two groups. These findings suggest that vitamin D supplementation may ameliorate symptoms of MDD, particularly in females, via a serotonin-dependent mechanism.